1. Field of the Invention
This invention is concerned with the crystallization of high silica ZSM-5 zeolite and, more particularly, is concerned with the preparation of high silica ZSM-5 zeolites having a silica-to-alumina ratio of greater than 70, up to and approaching infinity, by control of the pH so as to obtain zeolitic compositions having a rod-shaped morphology by controlling the final pH of the reaction medium to a range of 9.5-10.5.
2. Description of Prior Art
High-silica zeolites are extremely well known in the art and have been the subject of much attention in both the patent and technical literature. It has now been discovered that the morphology of the crystals produced from a forming solution vary depending upon the final pH of the reaction medium. As is well known in the art of the synthesis of zeolites, it is extremely difficult to control the pH and, in fact, it is extremely difficult to even measure the pH during the crystallizaton since crystallization is usually carried out in closed vessels under autogeneous pressure such that exact measurements are not practical. It is known in the art that there is a certain pH range over which ZSM-5 type zeolites can be prepared and, in this connection, U.S. Pat. No. 4,061,724 discloses at column 3 the pH range of 10-14. However, this patent does not teach that the morphology of the resulting crystals is dependent on the pH of the reaction medium. It is also known that the pH is difficult to control and that it varies during the course of crystallization.
Rod-shaped silica polymorphs are disclosed in U.S. Pat. No. 4,073,865. These materials are stated to be a novel silica polymorph and they require, for their preparation, the presence of a fluoride ion in the reaction media. The resulting rod-shaped products are stated to be extremely hydrophobic thereby making it particularly advantageous in absorption separations requiring minimum water absorption or at least interference from water vapor and the absorption of less polar molecules. Preparation of the materials involving the use of a fluoride ion has many disadvantages including disposal problems in connection with the waste liquids. Similarly, the off gas resulting from the calcination of the silica materials contains hydrogen fluoride and silica tetrafluoride thereby preventing releasing to the atmosphere until suitable purification techniques are employed.
Accordingly, it is the object of this invention to produce high silica ZSM-5 in the absence of a fluoride from a reaction medium which does not contain a fluoride ion said ZSM-5 being rod-shaped and having a high degree of hydrophobicity. It is noted that the production of rod-shaped ZSM-5 from a fluoride free media is not novel per se and, in fact, Example 27 of U.S. Pat. No. 3,702,886 specifically teaches the same. However, instant invention represents an improvement over the process toward said Example in that the products obtained by the process of this invention are more uniformly rod-shaped than those in said Example 27.